Complexes iron -phytic acid

Phosphate is better used when supplied by food than when supplied by phosphate salts, such as potassium phosphate. The food phosphate occurs largely as phosphate esters and, with gradual hydrolysis, enters the body relatively slowly and is used efficiently in the metabolic processes. Phosphate supplied as inorganic phosphate is rapidly absorbed, with a tendency to be excreted in the urine rather than to be used by tissues (Schuette and Linkswilcr, 1962). The phosphate in bran occurs largely in the form of phytic acid (inositol hexaphosphate). The phosphate groups of fhis compound may be only partially hydrolyzed in the gut- Phytic acid tends to be excreted via the fecal route as a complex with iron or calcium. 

The results of dietary zinc analysis need to be considered in terms of the availability of the zinc in the food for intestinal absorption. The zinc content of whole meals and the total daily zinc intake are not sufficient information on their own, without knowledge of factors which inhibit or promote intestinal absorption (O Dell, 1984). Free ionic zinc probably does not exist in the intestinal tract, zinc being bound to molecular species such as protein, amino acids, phytic acid, citrate and others. The bioavailability of the metal is determined by the nature of these zinc binding ligands. When the zinc complex is insolubie as in Zn-phytate, the uptake from diet is poor, whereas zinc-protein or zinc-amino acid complexes are more easily dissociated and are a good source of available zinc. Other dietary components affect zinc absorption such as the amount of iron, calcium and phosphate. 

Phytic Acid. Recent reviews (67,68,69) summarized the literature covering the relationship between phytic acid and mineral bioavailability in soy protein products. The formation of phytate-proteln-mineral complexes (particularly zinc chelates in flours, concentrates, and Isolates prepared from mature soybeans) may be responsible for reduced mineral availability. However, the iron in Fe-labeled mature soybeans is more available to iron-deficient rats than the iron in green-immature soybeans, even though mature soybeans contain three times more phytic acid (70). The factor(s) responsible for this difference in bioavallablllty has not been identified. 

Phytic acid and camosine (histidine-containing dipeptide), obtained from cereal and meat by-products, are effective inhibitors of hpid oxidation by several mechanisms, including metal inactivation and free radical quenching. Uric acid obtained from the decomposition of adenosine triphosphate in muscle also inhibits lipid oxidation by the same mechanisms. However, the importance of uric acid as an endogenous antioxidant in muscle foods is not clear. Various protein concentrates from soybeans, cottonseed and peanuts inhibit hpid oxidation in muscle foods. In addition to their iron binding activity, these crade extracts contain complex polyphenolic flavonoids that have potent antioxidant activity. 

Diets based on unleavened wheat bread contain a relatively large amount of phytic acid (inositol hexaphosphate), which can bind calcium, iron and zinc to form insoluble complexes that are not absorbed. Phytases in yeast catalyse dephosphorylation of phytate to products that do not chelate the minerals. 

Calcium forms stable insoluble salt with oxalic acid (see Section 10.2.3.2). In plant cells with higher concentrations of oxalic add, caldum oxalate can be actually present in the form of crystals. Some plants have been shown to bind metals in mixed complexes. For example, chromium can be bound in an oxalate-malate complex, and nickel and zinc can form a dtrate malate complex. Citric add has been proven to be a low molecular weight zinc ligand in human milk, and in casein micelles it binds calcium. It is also used as a food additive (acidulant, synergist to antioxidants and sequestrant), so great attention has been paid to the formation of its complexes with metal ions. The addition to cereal products leads to increased solubihty of naturally present iron, due to its release from phytic acid salts (phytates). 

The predominating form of iron in animal tissues is haem (particularly myoglobin and haemoglobin). In egg white, iron is bound in conalbumin, and in the yolk to phosphoprotein phosvitin. Milk contains the iron metaUoprotein lactoferrin and part of the iron is bound to casein. Conalbumin and lactoferrin are structurally similar to serum transferrin. In plants, iron is bound in various complexes, especially with phytic acid, aUphatic hydroxycarboxyhc acids, aminocarboxyhc acids, thiols, phenolic substances, nucleotides, peptides and proteins. The iron content in selected foods is shown in Table 6.8. Foods rich in iron are offal dishes, meat, eggs, pulses, tea and cocoa. Moderate amounts of iron are found in fish, poultry, cereals, spinach, parsley and nuts. Low levels of iron are present in milk, dairy products, fats and oils, potatoes and most fruit. 

In animal cells, it occurs as a component of phospholipids, substances containing phosphorus, fatty acids, and nitrogenous bases. In plant cells, it is found as phytic acid, an organic acid that binds calcium, iron, and zinc in an insoluble complex and interferes with their absorption. 

---